This invention relates to letterpress relief printing plates and a method for making such plates. More particularly, this invention relates to projection imaged letterpress relief printing plates.
In the printing art, time is of the essence. In the past, relief printing plates were usually made by photoengraving and etching a layer of zinc or magnesium metal. However, these methods required relatively long etching periods until sufficient relief was obtained on the metal plate. More recently, the art has disclosed that polymeric materials may be substituted for metal in the formation of printing plates. Modern letter-press printing now commonly utilizes printing plates having photographically produced relief images from photopolymer compositions. The photopolymer composition to be imaged can be a liquid or a solid. In the case of the solid photopolymer, the desired relief image is obtained by exposing the solid photopolymer bonded to a metal or plastic support sheet to light radiation projected through a photographic negative in contact with said solid photopolymer for a sufficient time to insolubilize the photopolymer composition in the image exposed parts. In the case of the liquid photopolymer, the desired relief image is obtained by exposing the liquid photopolymer cast directly on a support sheet to collimated light radiation projected through a photographic negative out of contact with said liquid photopolymer for a sufficient time to substantially cure the photopolymer composition in the image exposed areas. The relief printing plate is then developed by removing from the support sheet portions of photopolymer composition which were unexposed during the image exposure step.
Regardless of whether a liquid photopolymer or a solid photopolymer was used previously to make a relief printing plate, the image was projected through a photographic negative at a ratio of 1:1 with the photopolymer. In other words, the cured image on the photopolymer layer was the same size as the image in the photographic negative. Only with planographic or offset printing plates or printed circuit boards utilizing dry film (all which use a very thin layer of photopolymer) was an image projected through a photographic negative at a ratio greater than 1:1 (the size of the cured image on the photopolymer was greater than the size of the image in the photographic negative). There are many benefits which accrue from projection platemaking at a ratio greater than 1:1, e.g. reduction of material costs, reduction of storage requirements and providing of greater flexibility.
Accordingly, it is a principal object of this invention to provide a system for the projection imaging of letter-press relief printing plates at a projection ratio greater than 1:1.
It is another object of this invention to provide such a system which will drastically reduce materials costs by eliminating the use of full-size photographic film.
It is a further object of this invention to provide such a system which will dramatically reduce storage requirements.
It is a further object of this invention to provide such a system which will provide greater flexibility with regard to using the same roll of film to image different size relief printing plates.
It is yet a further objective of this invention to provide better quality printing plates than those made using conventional processes.